chore(test): setup test coverage with c8

[caugner]

Description

Sets up test coverage with c8, and adds a test:coverage script.

Motivation

Make it easy to identify missing test coverage.

Additional details

See this test job run to see how it looks.

Reasons for using c8 instead of experimental Node-native coverage reporters:

• c8 allows defining file excludes
• c8 allows excluding parts of files (see this GitHub search in BCD)
• c8 creates multiple reports simultaneously (1. text summary, 2. lcov.info, 3. index.html with visual), whereas you'd need to run node --test twice to get both
• c8 includes files without any coverage, whereas node --experimental-test-coverage omits these (see Report 0% coverage for untested files in a project (i.e., files matching some glob) nodejs/node#58887)
• performance is comparable:

  node --test --experimental-test-coverage  0.92s user 0.18s system 100% cpu 1.091 total
  node --test --experimental-test-coverage --test-reporter=lcov   0.93s user 0.11s system 148% cpu 0.702 total
  npx c8 node --test  0.98s user 0.21s system 112% cpu 1.053 total
  

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